Main Exchange Line And Cryogenic Distillation Air Separation Unit Incorporating Such An Exchange Line

ABSTRACT

A unit for separating air using cryogenic distillation is provided.

The present invention relates to a unit for separating air using cryogenic distillation.

It is commonplace to use aluminum brazed-plate exchangers for cryogenic exchangers.

For small-sized units, the costs of manufacture are high. This technology is mainly suited to “custom” units, “custom” solutions however not being a justifiable option for small-sized devices where the cost of the unit is of more importance that its performance.

Likewise, “custom” solutions generally result in extremely long manufacturing lead-times.

These exchangers nonetheless have the advantage of allowing heat to be exchanged between several fluids.

According to the invention, the main exchange line is made up of a set of standardized brazed stainless steel plate-type exchangers which can be installed in a conventional or vacuum perlite insulation cold box.

Each plate-type exchanger exchanges heat between two fluids at most.

Exchangers of this type (for example supplied by Tranter Inc., Swep or Alfa Laval) allow heat to be exchanged between two fluids, namely a hot fluid and a cold fluid.

In a cryogenic nitrogen generator, there are generally three fluids:

-   -   a hot fluid: air     -   two cold fluids: nitrogen and vaporized rich liquid (LRV).

To use these exchangers the stream of air will be split into two (ideally pro rata the flows of cold fluids or alternatively pro rata the thermal charge of each of the fluids) so that the overall heat exchange is performed in two parallel exchanges:

-   -   one air/nitrogen exchange     -   one air/vaporized rich liquid exchange.

The flow of air to the two exchangers may be split using equalizing valves, or naturally if the pressure drops of the air circuit in the two exchangers are balanced.

This splitting of the air stream into two has no impact on the overall performance of the exchange because the initial exchange diagram is nearly parallel, with no intermediate outlet. With the correct split of air stream, two exchanges each of which has a parallel exchange diagram can be performed in parallel.

Furthermore, the length of these exchangers is fairly limited (generally less than 1 m), which is quite short for achieving the thermal gradient of about 200° C. between the hot end and the cold end of the exchange, for a small temperature difference (ΔT logarithmic mean between 2 and 10° C.).

That entails the use of multiple-pass heat exchangers and/or the use of several exchangers in series in order to perform the heat exchange. One object of the invention is to provide an exchange line designed to be incorporated into an air separation unit, comprising at least two exchange assemblies, the assemblies being connected in parallel, each assembly comprising at least two exchange bodies, preferably at least three exchange bodies, connected in series with one another, means for sending a first flow that is to be cooled to the first assembly, so that the first flow is cooled down in each exchange body of the first assembly, means for sending a second flow that is to be cooled to the second assembly, so that the second flow is cooled down in each exchange body of the second assembly, means for sending a first fluid that is to be heated to a first of the exchange assemblies, so that the first fluid is heated up in each exchange body of the first assembly and means for sending a second fluid that is to be heated to a second of the exchange assemblies, so that the first fluid is heated up in each exchange body of the second assembly, characterized in that there is no means that allows some of the first flow to cool down in the second assembly, that there is no means that allows some of the second flow to cool down in the first assembly, that there is no means that allows some of the first fluid to heat up in the second assembly and that there is no means that allows some of the second fluid to heat up in the first assembly.

For preference, the exchange line comprises

-   -   means for separating a flow that is to be cooled downstream of         the two assemblies to form the first and second flows so that         the first and second flows that are to be cooled pass in series         through each exchange body of their respective assembly,     -   just two exchange assemblies.

The bodies of one assembly may be aligned in the direction of their length, arranged with their length parallel to that of the adjacent bodies, arranged with their length parallel to that of the adjacent bodies but in a staggered configuration.

Each exchange body may be a stack of rectangular plates (possibly with rounded corners) or stack of irregular hexagonal plates, connecting elements being positioned on each side of each body to connect it to the adjacent body.

At least one of the exchange bodies is a standardized exchanger, preferably in which at least two of the exchange bodies are of the same model.

Another object of the invention is to provide an air separation unit comprising a distillation column having a top-end condenser, an exchange line as claimed in one of claims 1 to 9, means for sending compressed and purified air to each of the exchange assemblies, means for withdrawing nitrogen gas from the column, means for sending the nitrogen gas to a first exchange assembly, means for sending the column bottom liquid to the condenser, means for drawing vaporized liquid from the condenser and means for sending the vaporized liquid to the second assembly.

The unit may comprise means for regulating the amount of air sent to the exchange assemblies.

At least two exchange bodies of one and the same assembly are placed one above the other, and are preferably all one above the other.

Another object of the invention is to provide a method of cooling air in an exchange line, comprising at least two exchange assemblies, the assemblies being connected in parallel, each assembly comprising at least two exchange bodies, preferably at least three exchange bodies, connected in series with one another, means for sending a first flow of air that is to be cooled to the first assembly, so that the first flow is cooled down in each exchange body of the first assembly, means for sending a second flow of air that is to be cooled to the second assembly, so that the second flow is cooled down in each exchange body of the second assembly, means for sending a first fluid that is to be heated to a first of the exchange assemblies, so that the first fluid is heated up in each exchange body of the first assembly and means for sending a second fluid that is to be heated to a second of the exchange assemblies, so that the first fluid is heated up in each exchange body of the second assembly, characterized in that all of the first flow is sent only to the first assembly, all of the second flow is sent only to the second assembly, all of the first fluid is sent only to the first assembly and all of the second fluid is sent only to the second assembly.

Possibly, the first and second flows enter their respective assemblies at different pressures and/or different temperatures.

Possibly, the first and second flows are combined downstream of the first and second assemblies.

The invention will be described in greater detail with reference to the figures.

FIG. 1 depicts an exchange line according to the invention,

FIGS. 2 to 5 depict an exchange assembly suited to forming part of an exchange line according to the invention, and

FIG. 6 depicts an air separation unit according to the invention.

In FIG. 1, a heat exchange line according to the invention consists of two exchange assemblies, each comprising three exchange bodies. For preference, the six exchange bodies may be identical and are standardized exchangers of the same model by a single manufacturer. Each exchange body exchanges heat between just two fluids. In use, the exchange bodies 13A to 13C are positioned one above the other, 13C being the lowermost and 13A being the uppermost. Likewise, in use, the exchange bodies 11A to 11C are placed one above the other, 11C being the lowermost and 11A the uppermost.

Air to be cooled 1 is split into two using two valves 7, 9 to form two flows 3, 5. The flow 3 is sent to the first assembly consisting of the exchange bodies 11A, 11B, 11C connected in series and is not sent to the second assembly. The flow 5 is sent to the second assembly consisting of the exchange bodies 13A, 13B, 13C connected in series and is not sent to the first assembly. The air flows are not combined except possibly downstream of the last bodies 11C, 13C. The first and second flows enter their respective assemblies at different pressures and/or different temperatures having, for example, been compressed to different pressures.

A flow of vaporized rich liquid LRV 17 from the condenser of a simple column is sent to the bodies 11C, 11B, 11A in that order in order to be heated up to ambient temperature. A flow of nitrogen 19 from the same column is sent to the bodies 13C, 13B, 13A.

One of the assemblies or both of the assemblies may be replaced by an assembly like that illustrated in FIGS. 2 to 5.

In FIGS. 2 and 3, the inlet to the exchange body is situated on the length of the body. It is therefore possible to conceive of arranging the bodies with their lengths in a staggered configuration as illustrated in FIG. 2, connected by pipes 31 perpendicular to the length axis of the bodies. In use, the body 13A is above the body 13C, and the body 13B is above the body 13D. Otherwise, as in FIG. 3, the lengths are parallel to one another, possibly with insulating blocks 25, perpendicular to the length axis of the body, placed between the bodies 13. The bodies 13 are connected by pipes 31 perpendicular to the length axis of the bodies. The bodies are, in use, positioned one above the other, 13A being the uppermost and 13E the lowermost.

The bottom body 13E is angled because there is a risk of having droplets of liquid air at the outlet.

In FIG. 4, the bodies 11 are arranged as in FIG. 1.

FIG. 5 shows the special arrangement suited to bodies of irregular hexagonal cross section, such as MAXCHANGERs®. The bodies 11A, 11B, 11C are arranged one above the other with connecting elements 21 filling the gaps on each side of the bodies to form an element of rectangular overall cross section capped by four fluid inlet/outlets 23. This monolithic element is particularly easy to install and to support.

Even if this type of exchanger gives the impression of making the arrangement more complicated, it does allow the use of inexpensive standard exchangers with extremely short manufacturing lead-times: the cryogenic exchangers therefore no longer fall on the critical path when producing the air separation unit.

FIG. 6 shows an air separation unit for producing nitrogen comprising an exchange line according to the invention. The line illustrated is that of FIGS. 1 and 4 but obviously those of FIGS. 2, 3 and 5 could have been incorporated in the same way. The air 3, 5 from the two groups of exchangers is sent to the column 33. Nitrogen 19 is withdrawn at the top of the column and sent to the group 13 and vaporized rich liquid LRV 17 from the top-end condenser 35 is sent to the second group 11. 

1-15. (canceled)
 16. An exchange line designed to be incorporated into an air separation unit, comprising: at least two exchange assemblies, consisting of a first and of a second assembly, the assemblies being connected in parallel, each assembly comprising: at least two exchange bodies, connected in series with one another, means for sending a first flow that is to be cooled to the first assembly, so that the first flow is cooled down in each exchange body of the first assembly, means for sending a second flow that is to be cooled to the second assembly, so that the second flow is cooled down in each exchange body of the second assembly, means for sending a first fluid that is to be heated to the first assembly, so that the first fluid is heated up in each exchange body of the first assembly, and means for sending a second fluid that is to be heated to a second assembly, so that the second fluid is heated up in each exchange body of the second assembly, wherein: there is no means that allows some of the first flow to cool down in the second assembly, there is no means that allows some of the second flow to cool down in the first assembly, there is no means that allows some of the first fluid to heat up in the second assembly, and there is no means that allows some of the second fluid to heat up in the first assembly.
 17. The exchange line of claim 16, wherein each assembly comprises at least three exchange bodies.
 18. The exchange line of claim 16, further comprising means for separating a flow that is to be cooled downstream of the two assemblies to form the first and second flows so that the first and second flows that are to be cooled pass in series through each exchange body of their respective assembly.
 19. The exchange line of claim 16, comprising two exchange assemblies.
 20. The exchange line of claim 16, in which the bodies of one assembly are aligned in the direction of their length.
 21. The exchange line of claim 16, wherein the bodies of one assembly are arranged with their length parallel to that of the adjacent bodies.
 22. The exchange line of claim 16, wherein the bodies of one assembly are arranged with their length parallel to that of the adjacent bodies and in a staggered configuration.
 23. The exchange line of claim 16, wherein each exchange body is a stack of rectangular plates.
 24. The exchange line of claim 23, wherein the rectangular plates have rounded corners.
 25. The exchange line of claim 16, wherein each exchange body is a stack of irregular hexagonal plates, connecting elements being positioned on each side of each body to connect it to the adjacent body.
 26. The exchange line of claim 16, wherein at least one of the exchange bodies is a standardized exchanger.
 27. The exchange line of claim 26, wherein at least two of the exchange bodies are of the same model.
 28. An air separation unit comprising: a distillation column having a top-end condenser, an exchange line as claimed in claim 16, means for sending compressed and purified air to each of the exchange assemblies, means for withdrawing nitrogen gas from the column, means for sending the nitrogen gas to a first exchange assembly, means for sending the column bottom liquid to the condenser, means for drawing vaporized liquid from the condenser, and means for sending the vaporized liquid to the second assembly.
 29. The unit of claim 28, further comprising means for regulating the amount of air sent to the exchange assemblies.
 30. The unit of claim 28, in which at least two exchange bodies of one and the same assembly are placed one above the other,
 31. The unit of claim 30, wherein at least two exchange bodies of one and the same assembly are all one above the other.
 32. A method of cooling air in an exchange line, the exchange line comprising: at least two exchange assemblies, the assemblies being connected in parallel, each assembly comprising at least two exchange bodies, connected in series with one another, means for sending a first flow of air that is to be cooled to the first assembly, so that the first flow is cooled down in each exchange body of the first assembly, means for sending a second flow of air that is to be cooled to the second assembly, so that the second flow is cooled down in each exchange body of the second assembly, means for sending a first fluid that is to be heated to a first of the exchange assemblies, so that the first fluid is heated up in each exchange body of the first assembly, and means for sending a second fluid that is to be heated to a second of the exchange assemblies, so that the first fluid is heated up in each exchange body of the second assembly, the method comprising: sending all of the first flow only to the first assembly, sending all of the second flow only to the second assembly, sending all of the first fluid only to the first assembly, and sending all of the second fluid only to the second assembly.
 33. The method of claim 32, wherein the first and second flows enter their respective assemblies at different pressures and/or different temperatures.
 34. The method of claim 32, wherein the first and second flows are combined downstream of the first and second assemblies. 